LEDs convert electrical energy into optical energy. In semiconductor LEDs, light is usually generated through recombination of electrons, originating from an n-type doped semiconductor layer, and holes originating from a p-type doped semiconductor layer. In some infra-red emitting semiconductor materials light can be generated by electron intersub-band transitions rather than electron hole transitions. Herein, the area where the main light generation takes place is termed the light-emitting layer.
Further, the term “light” is used herein in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
A major challenge is to extract as much of the emitted light as possible from the semiconductor material into the surrounding medium, typically air, thereby increasing the extraction efficiency. This is hindered by total internal reflection at the surfaces of the semiconductor. This challenge is even greater in the field of μLEDs.
As used herein, the term “extraction efficiency” (EE) encompasses the amount of light extracted from an LED device as a proportion of the total light generated by the device. The EE may be expressed as a percentage. Further, the term “μLED” is used herein to encompass an LED that is smaller than a standard cuboid LED. A μLED may have an active region of approximately 10 μm or greater. Further, a μLED may comprise a mesa structure configured to direct light emitted from the active region to an emission surface in a quasi-collimated fashion.
A common approach to improve the EE of LEDs is to roughen the surfaces where the light exits the chip. This reduces the amount of light trapped by total internal reflection that occurs by randomising the angles at which the light hits the surface.
It is desirable to increase the EE of LEDs and μLEDs.